ghostcript: work around a parallel check problem

http://hydra.nixos.org/build/24811657
Hopefully OK now - I couldn't reproduce it with 4 cores and HT.

pkgs/misc/ghostscript/default.nix

@@ -77,6 +77,8 @@ stdenv.mkDerivation rec {
     ];
 
   doCheck = true;
+  preCheck = "mkdir ./obj";
+  # parallel check sometimes gave: Fatal error: can't create ./obj/whitelst.o
 
   # don't build/install statically linked bin/gs
   buildFlags = "so";